This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 675–680 675 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 675–680 Photodissociation of isobutene at 193 nm Gabriel M. P. Just, ab Bogdan Negru, ab Dayoung Park ab and Daniel M. Neumark* ab Received 19th August 2011, Accepted 28th October 2011 DOI: 10.1039/c1cp22651g The collisionless photodissociation dynamics of isobutene (i-C 4 H 8 ) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H+C 4 H 7 and CH 3 + CH 3 CCH 2 . Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH 3 loss channel lies 13 kcal mol 1 higher in energy, the CH 3 : H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations. I. Introduction Isobutene, i-C 4 H 8 , (2-methylpropene) is the smallest branched alkene. It plays a key role in combustion chemistry as an intermediate in the pyrolysis of iso-octane and in the oxidation of fuel additives such as MTBE and ETBE (methyl and ethyl t-butyl ether). 1 The chemistry of isobutene in the Earth’s troposphere, notably its reactions with NO 2 and NO 3 , 2,3 is of interest, as are its reactions with free radicals in the atmosphere of Titan in order to form larger hydrocarbons. 4 Isobutene has been implicated as a product in the O( 3 P)+t-C 4 H 9 (t-butyl) radical–radical reaction 5 and from the photodissociation of tert-C 4 H 9 . 6,7 However, the photodissociation of isobutene itself has not been reported previously. In this paper, we investigate the collisionless photodissociation of isobutene at 193 nm in order to gain new insights into its unimolecular photochemistry and dissociation dynamics. The work presented here is motivated by numerous studies of the bimolecular and unimolecular kinetics of isobutene in shock tubes and flames. 8–13 These studies have focused on elucidating the mechanisms for the oxidation and pyrolysis of isobutene. An issue arising from this body of work is the identity of the products arising from the unimolecular decay of isobutene. In some kinetics studies, reaction mechanisms are proposed in which only H atom loss is included, 10,12 whereas others also include the somewhat higher energy CH 3 loss channel. 8,13,14 Photodissocia- tion measurements provide unambiguous identification of the primary products from photoexcitation to an excited electronic state. In cases where dissociation proceeds via internal conversion to the ground state followed by statistical decay, the results of photodissociation experiments can have direct bearing on the interpretation of kinetics experiments in which it is often very difficult to identify product channels for specific reactions. Photodissociation of isobutene is also of interest in light of previous work by Zierhut et al. 6 and our group 7 on the photodissociation of the t-C 4 H 9 radical near 248 nm. One concern in those experiments was that some observed channels were from the photodissociation of vibrationally hot isobutene produced in the pyrolysis source used to generate t-butyl radical rather than from t-butyl itself. An independent study of isobutene photodissociation could thus corroborate the interpretation of the previous experiments on t-butyl. The UV absorption spectrum of the isobutene molecule begins around 205 nm and comprises numerous closely spaced bands; 15,16 the band around 193 nm has been assigned to the lowest-lying pp* transition. 17,18 Excitation at 193 nm can lead to photodissociation by two bond cleavage channels involving loss of either an H atom or a CH 3 group: (CH 3 ) 2 CCH 2 + hn - CH 2 CH 3 CCH 2 +H DH 0 = 88.3 kcal mol 1 , 19 (1) (CH 3 ) 2 CCH 2 + hn - CH 3 CCH 2 + CH 3 DH 0 = 100.9 kcal mol 1 , (2) The 2-methylallyl radical from channel 1, can further dissociate to form allene (C 3 H 4 ) via the loss of a methyl group. 20 The barrier to this dissociation process has been calculated by Li et al. 21 to be 55.5 kcal mol 1 . Furthermore, the 2-propenyl radical from channel 2 can undergo a 1,2-hydrogen shift over an isomerization barrier of 45.4 kcal mol 1 to form the allyl radical. 22 Fig. 1 shows the primary energetics for channels 1 and 2 as well as the barrier heights and energies for subsequent dissocia- tion and isomerization. In this work, we investigate the collisionless photodissociation of isobutene at 193 nm using molecular beam photofragment translational spectroscopy. This experiment yields the kinetic energy and angular distribution for each photofragmentation channel, enabling the direct identification of the primary photo- fragments and providing insight into the dissociation mechanism. a Department of Chemistry, University of California, Berkeley, CA 94720, USA. E-mail: [email protected]b Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by University of California - Berkeley on 17 January 2012 Published on 28 November 2011 on http://pubs.rsc.org | doi:10.1039/C1CP22651G View Online / Journal Homepage / Table of Contents for this issue
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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 675–680 675
This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 675–680 679
For channel 2, the C–C bond fission products are CH3 +
CH3CCH2, the 2-propenyl radical. As seen in Fig. 1, this
species lies 20.6 kcal mol�1 above the allyl radical,22 and one
must then consider the possible role of allyl in the dissociation
dynamics. For example, it is possible for allyl to be formed
directly from the dissociation of isobutene through a multi-
center transition state, in which an H atom from the remaining
CH3 group transfers to the central C atom as the methyl
fragment departs. Such a transition state typically results in a
substantial exit barrier along the reaction coordinate which
would then lead to a translational energy distribution peaking
well away from ET = 0, in contrast to the distribution in
Fig. 7. Hence concerted production of allyl seems unlikely.
It is also possible for 2-propenyl products formed with more
than 45.4 kcal mol�1 internal energy to isomerize to allyl by a
1,2-hydrogen shift, according to the energetics in Fig. 1.
However, since ETmax for channel 2 is only 47 kcal mol�1 at
193 nm, this means that isomerization can only occur for
dissociation events with ET o 1.6 kcal mol�1, which corres-
ponds to a very small fraction of the P(ET) distribution in
Fig. 7. Moreover, this value of ET assumes no internal excita-
tion of the CH3 fragment. Isomerization to allyl therefore
represents a very minor contribution to the overall dynamics
and may not occur at all.
It thus appears that our channel 2 : channel 1 branching ratio of
1.7 (5) favors the higher energy C–C bond fission channel over
C–H bond fission, a somewhat unexpected result at first glance.
However, as discussed in previous work on isobutene kinetics,13,14
the A-factor for CH3 loss is higher than for H loss because CH3
loss results in three more rotational degrees of freedom at the
transition state than H loss. As a result, CH3 loss should become
faster than H loss at a sufficiently high temperature.
We have explored this effect from the perspective of our
experiment by calculating microcanonical rate constants ki(E)
for the two channels with RRKM theory,29
kiðEÞW�ðE � E0Þ
hrðEÞ ð6Þ
Here W* defines the total number of states of the critical
configuration, E0 is the energy of the transition state, and r(E)denotes the density of states of the reactant at total energy E.
The density and sum of states were calculated by direct state-
count method using the Beyer-Swinehart algorithm,30,31 using
vibrational frequencies obtained from the electronic structure
calculations described in Section IV. Vibrational frequencies
for all modes perpendicular to the reaction coordinate were
calculated for both bond fission channels. For rCC 4 2.6 A,
we treated torsional motion of the methyl group as a one-
dimensional free rotor with a rotational constant of around
5 cm�1.
By looking at the evolution of the calculated rate constants
for both channels as a function of fragment separation, we found
minimum values of the rate constants for channels 1 and 2 at
rCH = 3.4 A and rCC = 3.4 A, respectively. These values were
kCH3= 1.55 � 108 s�1 and kH = 8.42 � 107 s�1, leading to a
theoretical branching ratio of CH3 loss :H loss of 1.8, in
remarkably close agreement with experiment. We also calcu-
lated the branching ratio as a function of excitation energy
at these two transition state geometries as shown in Fig. 8. At
193 nm excitation (148 kcal mol�1), methyl loss dominates
over H loss. However, the H loss channel becomes more
important with decreasing photon energy (i.e. increasing wave-
length) to become the dominant channel at about 133 kcal mol�1
(215 nm), which is below the 205 nm onset15 of the isobutene UV
absorption spectrum. In any case, our RRKM results are
certainly consistent with the arguments put forth in kinetics
papers regarding the A-factors for the two bond fission
channels and suggest that both channels should be considered
when constructing kinetic models for isobutene pyrolysis and
oxidation.
VI. Conclusions
We have investigated the photodissociation dynamics of the
isobutene molecule at 193 nm using photofragment transla-
tional spectroscopy. The translational energy distribution and
the product branching ratio between H and CH3 loss were
obtained. The translational energy distributions indicated that
both channels take place via statistical dissociation on the
ground state potential energy surface. The branching ratio
between both channels was determined experimentally to be
1.7(5) in favor of the higher energy CH3 loss channel. Electronic
structure calculations combined with RRKM theory showed
that such a result is consistent with statistical dissociation at the
excitation energy used in our experiment.
Acknowledgements
This work was supported by the Director, Office of Basic Energy
Sciences, Chemical Sciences Division of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231.
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